Endo Orders FT4 and FT3, AND Total T4 and Total T3; Reason Given Is Mysterious

[Bird_42]

Hello. I am a 38-year-old male, and I live in Chicago, IL. I have autoimmune hypothyroidism.

A few months ago, my endo moved to a different state, and his successor in the clinic is my
new endo. I will be meeting with this new endo for the first time in about a month. She has
ordered some thyroid blood tests, and I will have those tests about a week before the meeting
so that we can discuss the results at the meeting.

In addition to ordering the TSH, Free T4, and Free T3 tests, she has also ordered the Total T4
and Total T3 tests. Now, everything that I've read indicates that the Total T4 and Total T3 tests
are useless, and I asked her why she is ordering these tests.



She replied as follows:

I typically will get the total T3 and T4 results in addition to the free T3/free T4 if there is a question
of lab variation in the "free level".



I asked her to elaborate on this. I asked her what "variation" she was talking about.



She replied as follows:

Sometimes certain medications/illnesses, changes in binding protein, can alter the free T4/T3 concentrations.



I am STILL not sure what she's talking about. :)   Can someone on this forum please interpret what she said?



Thank you.

[Red_Star]

Hi Bird_42.

Yes i did mean to say increased TBG increases total T4 and total T3 and decreased TBG decreases total T4 and total T3.  Glad you didn't spot the TGB mistake....wait ummmm. :)  Note to self: try to proof read answers when typing in the early hours of the morning. lol

In normal conditions, free T4 and free T3 exert direct and indirect negative feedback on TSH levels.  This results in tight regulation of thyroid hormone production and constant levels of free T4 and free T3 regardless of TBG levels.

[Bird_42]

Hi, Red_Star.

I've done some google-searching on TBG, and I think you meant to say that increased TBG *increases* Total T4 and Total T3 (without affecting Free T4 and Free T3), and that decreased TBG *decreases* Total T4 and Total T3 (without affecting Free T4 and Free T3).



However, what confuses me is *why* TBG affects Total T4 and Total T3, without affecting Free T4 and Free T3.

Logic would dictate that, if TBG increases, the amount of T4 that is bound will increase, and the amount of Free T4 will decrease, and the amount of Total T4 will remain constant.

However, if an increase in TBG increases Total T4 without affecting Free T4, then that means that an increase in TBG somehow causes new T4 to be created.

So, what am I missing here?

[Red_Star]

The binding protein is known as thyroid binding globulin (TBG).  

Increased TBG reduces total T4 and T3. Increased TBG levels may be due to hypothyroidism, liver disease, and pregnancy.

Decreased TBG elevates total T4 and T3.  Decreased TBG levels may be due to hyperthyroidism, renal disease, liver disease, severe systemic illness, Cushing syndrome, medications, and malnutrition.

TGB does not affect free T4 and free T3.

In patients with TBG deficiency, labs will show normal TSH and free T4 but low total T4 and occasionally low total T3.

[Brucergoldberg]

Sounds like she/he is old school and has some agenda.  I have had no less than 20 of these tests done in the last 3 years by 12 different doctors and have never had total done.